1. Field of the Invention
The present invention relates to a charging method for a battery pack that is a series connection of a plurality of lithium ion rechargeable batteries, and in particular relates to a charging method that charges while protecting a particular battery when its battery voltage becomes high.
2. Description of the Related Art
In a lithium ion rechargeable battery pack, a plurality of batteries are connected in series to raise the output voltage. This battery pack is fully charged by charging with constant current until battery voltage reaches a specified voltage, then subsequently charging with constant voltage to avoid rise in battery voltage. Since battery pack batteries are connected in series, the same charging current flows through all the batteries. If the electrical characteristics of all the series connected batteries are the same, battery voltage will rise in the same manner in all batteries. However, as the batteries degrade with repeated charging and discharging, imbalance will develop between the electrical characteristics of each battery. This is because not all batteries degrade in exactly the same fashion. For batteries with unbalanced electrical characteristics, even if they are charged with the same current, voltage differences can result. In particular, the voltage of a degraded battery will increase. Here, while detecting total voltage, a battery pack is fully charged by constant current charging then switching to constant voltage charging. In a method that charges a battery pack while controlling charging via total voltage, there is no distinction between a particular battery with high voltage and other batteries in a low voltage state. As a result, voltage of a particular battery can become high and it can be detrimentally affected. To eliminate this drawback in prior art charging methods, voltage for each battery is monitored during charging, and charging is suspended if the voltage of any battery exceeds a maximum voltage. In this method, no battery voltage exceeds the maximum voltage. However, in this charging method, since charging is suspended when the voltage of one battery exceeds the maximum voltage, the capacity to which the battery pack can be charged decreases if battery characteristics become unbalanced. (It has the drawback that effective charge capacity of the battery pack is reduced if battery characteristic imbalance develops.) To eliminate this drawback, a charging method that fully charges each series connected battery independently is cited in Japanese Patent Application Disclosure 2003-157908.